PHYSIOLOGICAL ECOLOGY Effects of Cottonwood Leaf Beetle (Coleoptera: Chrysomelidae) Larval Defoliation, Clone, and Season on Populus Foliar Phagostimulants

The cottonwood leaf beetle, Chrysomela scripta F., is a serious defoliator of plantation Populus in the United States. Current control methods include biorational and synthetic chemicals as well as selecting Populus clones resistant or tolerant to C. scripta defoliation. SpeciÞc ratios of long-chain fatty alcohols to alpha-tocopherylquinone ( -TQ) on the leaf surface of Populus spp. act as phagostimulants to adult C. scripta. The chemical concentrations and ratios vary among Populus clones;however, theeffectofdefoliationon the subsequentproductionof thesechemicals isunknown. We investigated the effects of defoliation, clone, and season on Populus leaf surface chemical production. Chemical concentrations and ratios were monitored in 1998 and 1999 on eight Populus clones with and without larval C. scripta defoliation. Chemicals were extracted from the leaf surface andanalyzedviagas chromatography.LarvalC. scriptadefoliation rarelycausedchanges in leaf surface chemistry at the defoliation levels tested; however, the production of these phagostimulants did vary by clone and season. Foliar alcohol and -TQ concentrations and -TQ:total alcohol ratios differed signiÞcantly among clones. Furthermore, -TQ concentrations and -TQ:total alcohol ratios varied temporally in some, but not all, clones. In general, foliar alcohol and -TQ concentrations either did not vary or increased, but -TQ:total alcohol ratios declined throughout the growing season. This research illustrates that the production of leaf surface phagostimulants is not a function of defoliation, but ismost likelycontrolledbygenetic andphysiologicprocesses.Additionally, becausePopulusclones vary in their foliar chemistry, this variation could be exploited in tree breeding programs.